Why a separate reminders database?

Mac OS already has reminders, other applications, such as MailTags, take advantage of this and add features to the poorly featured default reminders app: I wonder why this app needs a custom and separate database, when it really doesnt add that much.
Besides this, the user interface is really good, but the application costs too much.
So, think twice before spending the money.

Skofic about Things, v2.0.1